"I've always said that the Garden Bridge is a great idea," he replied.

"The idea of a bridge for pedestrians which is green, eco-friendly and a garden is great.

"My criticism has always been that I'm not sure that it's the best use of taxpayers' money.

"I can think of a lot of better things to spend £30 million worth of taxpayers' money on that the Mayor is in charge of."

Mr Khan insisted that this week's deal – which has seen £20 million of Transport for London's funding for the Garden Bridge converted from a grant to a 50-year loan – is "a good deal for the taxpayers not just of Lambeth but of London".

The central Government grant of £30 million for the bridge is unaffected by the renegotiated deal.

We asked the Labour mayoral candidate whether he would meet opponents of the Garden Bridge from around the South Bank and Waterloo.

He said: "Of course. I will carry on doing what I have been doing since I have been selected as the candidate which is meeting as many Londoners as I possibly can.

"This is a good example of what a Labour candidate who has not been elected can do. Just imagine all the good I could do if I was elected next May."

We asked Lambeth leader Cllr Lib Peck for her response to the criticisms of the Garden Bridge project aired this week by Kate Hoey MP and the South Bank's three Labour councillors.

"I understand that people very close to the site are concerned about the impact of the bridge, but I think we've got to look at it in the round," she said.

"I think it could be a very creative, ambitious scheme for London that brings many benefits."

She underlined that her chief concern had been about the level of public funding to be ploughed into the project and said that this had been successfully addressed.

The council will now enter talks with the Garden Bridge Trust and Coin Street Community Builders with a view to granting a new lease for the green space on the South Bank where the bridge's southern landing building would be built.

"We will certainly listen to South Bank residents and work with their local councillors in terms of the land negotiation, but obviously a lot of the concerns sit around the planning conditions," she said, explaining that the borough's planning applications committee is due to examine some of the detail at meetings in December and January.

This week a Twitter poll carried out by the SE1 website showed that 85 per cent of respondents thought Cllr Peck was wrong to reopen talks with the Garden Bridge Trust.

"I've had a lot of letters both pro the bridge and against the bridge over the last few weeks," she told us.

We put it to Cllr Peck that turning a £20 million grant into a 50-year loan might be seen as tinkering at the edges in the context of the £175 million cost of the bridge and the remaining £40 million of public funding.

"I don't think it's very often that a public authority such as Transport for London reduces the grant it's committed to something on the basis of an intervention by a local council leader, so I consider that a success," she said.

The council leader refused to comment on the controversy over the procurement process run by Transport for London which led to the selection of Heatherwick Studio and Arup to work on designs for a footbridge between Temple and the South Bank.

Last month Boris Johnson claimed at Mayor's Question Time that there were "no issues whatever" with the procurement process, days before he published a written answer admitting that "the processes followed did not comply with all aspects of procurement regulation".

Artist Will Jennings – who organised the recent A Folly for London competition poking fun at the Garden Bridge – was unimpressed by Cllr Peck's remarks.

He told the SE1 website: "Lib mentions the concerns of people close to the site, but the criticism about the Garden Bridge I receive daily as organiser of A Folly For London is not just local interest.

"It is from leading experts in architecture, heritage, public finance, environmentalism, urban studies and more – all who have deep issues with the development.

"On top of that, taxpayers and lovers of London the world over have come to me with profound concerns."

Mr Jennings also took issue with Sadiq Khan's use of the terms 'green' and 'eco-friendly' to describe the bridge: " If Sadiq had responded to any of the many requests from A Folly For London to meet and discuss the legitimate concerns of huge numbers of experts and public then he may understand that the Green Party, London Wildlife Trust and more environmental organisations damn the development as greenwash and hugely damaging and of no benefit to Londoners or the city.

"Unfortunately, I fear Sadiq may be listening to the views of developers who will hugely benefit from increased land values than any of the citizens of London he seeks to represent from next May."

"He's merely facilitating Boris Johnson's vanity project whilst pathetically claiming it a triumph for a Labour candidate," she said. "A spectacular own goal which has cost him hundreds of voters."

Responding to Cllr Peck's comments, Ms Cheung added: "It's shocking that she still refuses to acknowledge the other major issues that the Garden Bridge would bring and is crowing about singlehandedly making the worst decision for her constituents."

During his monthly 'Ask Boris' LBC phone-in, Boris Johnson this week joked that if Lambeth had not agreed to co-operate with the Garden Bridge the project could have been turned into a 'Garden Pier' that stopped "five yards short of Lambeth".